A collision-prevention method, in particular for use in combination with a system for adaptive cruise control in motor vehicles, is described in the publication “Adaptive Cruise Control System—Aspects and Development Trends” by Winner, Witte, Uhler and Lichtenberg, Robert Bosch GmbH, in SAE Technical Paper Series 961010, International Congress & Exposition, Detroit, Feb. 26–29, 1996. The Adaptive Cruise Control (ACC) system described in this publication is based on a distance sensor, for example, a multi-target-capable radar sensor mounted on the front of the vehicle to measure distances and relative speeds of other vehicles driving in front. The speed of one's own vehicle is regulated as a function of the measured data from this radar sensor so as to maintain a desired preset distance from the vehicle driving directly in front, and the driver is able to preselect this desired preset distance in the form of a setpoint time window. If there is no vehicle traveling in front within the range of the radar, the speed is regulated at a desired speed set by the driver.
This system normally intervenes in the drive system of the vehicle via a throttle valve, for example, so that the vehicle speed is regulated on the basis of the driving torque of the engine. However, when greater deceleration of the vehicle is required, e.g., on a downward slope or due to the distance from the vehicle in front, and the engine drag is not enough to achieve adequate deceleration of the vehicle, then the system intervenes in the brake system of the vehicle.
Published German patent document DE 36 37 165 describes a control method in which the relative speed of approach of the object which is the source of the collision risk, the theoretical period of time until impact in the absence of response, and the vehicle deceleration required at that instant to reliably prevent the collision are calculated from the position data compiled and an adequate safety distance is determined, taking into account the possible braking deceleration of the vehicle. The extent to which the actual distance is less than this safety distance is a measure of the collision probability. Subsequently, the system responds in three stages as a function of the collision probability thus determined. In stage 1, just an acoustic or visual warning signal is output for the driver. In stage 2, there is automatic intervention in the brake system at a braking force calculated in advance. Finally, in stage 3, a braking operation at maximum braking force is triggered.
In these methods, however, there are problems in determining the criteria for triggering of stage 2 because of the unavoidable uncertainty in estimating the collision risk. If the threshold for triggering of this stage is set too high, collisions cannot be reliably prevented. However, if this threshold is lowered, it results in unnecessary braking maneuvers, because of an incorrect appraisal of the collision risk; the unnecessary braking may also have a negative effect on the comfort and feeling of safety of the occupants of the vehicle, while irritating the following traffic or even becoming the cause of rear-end collisions in the following traffic.
The object of the present invention is to reduce the frequency of unnecessary vehicle decelerations while maintaining a high collision safety.